This invention relates to systems for transmitting and receiving data by means of phase modulated RF signals.
The use of digital computers for business purposes can be greatly expanded where data can be transferred from one computer to another by the use of communications links. Very often such computer data is transmitted by means of tones over ordinary telephone lines. Telephone lines have a limited modulation bandwidth and may be subject to noise or other problems which renders them inappropriate for high speed data transmissions.
In addition to telephone lines, many localities are now equipped with cable television networks. Such networks are primarily adapted for transmission of television signals from a central location, called the "head end" of the network, to subscribers located at various locations in a geographic area. In addition to the "downstream" transmission of television signals, it is possible to provide both "upstream" and "downstream" transmission of data signals, so that the signals can be transferred between computers attached to terminals of the network. In such systems, data is transmitted upstream at a relatively low RF frequency, upon arriving at the system head end the data bearing signal is translated into a higher frequency channel, corresponding to an unused television channel, for transmission downstream on the cable to all of the subscriber terminals. In this way, data can be transferred from a first computer, located at one cable network terminal, up to the system head end and then downstream at a higher frequency to another computer located at another cable network terminal. This arrangement facilitates the interconnection of remotely located computers used for a common purpose, for example the computers located at the central office and at branch offices of a banking institution.
In other applications, data may be transferred by means of exclusively dedicated cable transmission lines, for example between computers located at various locations within an industrial plant. Also data may be transmitted at RF frequencies over long distances, for example between computers located in one city, via satellite relay, to computers located in another city. It should be recognized that the term "data" as used herein is not limited to computer generated signals, but is equally applicable to digitized voice, video or teletype signals.
All of the above described systems for transmitting data by means of an RF communications link require that the data be modulated onto an RF carrier signal and demodulated at the receiving terminal, so that it can be converted into a suitable format for use by the receiving computer. The devices for modulating and demodulating RF signals with data are commonly known as modems. Several schemes for modulating RF carriers with data are known, including pulse amplitude modulation, wherein the amplitude of the RF carrier during a selected interval is determined by the data state of the data to be transmitted; pulse duration modulation, wherein the length of a transmitted RF pulse is used to represent a particular data state; pulse code modulation, wherein pulses are transmitted in a code sequence to represent data states; frequency hop modulation, wherein different data states are represented by different transmitted frequencies; and phase-shift modulation, wherein different phase states of the RF carrier represent different data conditions. The present invention is related primarily to data transmission systems which use a phase-shift modulated carrier.
The generation of phase-shift modulated signals from logic signals is easily achieved by use of a modulating apparatus such as shown in FIG. 1. This circuit is useful for generating binary phase-shift modulated signals wherein data states are represented by the phase of the carrier which is either 0.degree. or 180.degree.. In this circuit, the CW output from oscillator 10 is modulated in mixer 14 by positive or negative voltage data representative signals supplied by source 12. The output of mixer 14 may be supplied to output 18 by amplifier 16. FIG. 2 shows typical data signals which may be applied to mixer 14 to result in a binary phase-shift modulated signal, which is shown in FIG. 3. For simplicity in illustration, the FIG. 3 signal is shown having a frequency equal to twice the data rate. This frequency selection can result in phase-shifts occurring at null crossings as illustrated. Those skilled in the art will recognize that usually the carrier frequency is significantly higher than the data rate, and phase shifts may occur at other than the zero crossings of the RF carrier.
One problem with the use of phase-shift modulated signals arises out of the need to reconstruct a phase-coherent unmodulated carrier in the receiver for use in demodulating the received signal. Most prior art demodulators makes use of a phase-locked loop in order to provide a regenerated carrier for use in demodulation. The phase-locked loop makes use of a voltage controlled oscillator whose frequency and phase is regulated by a locally generated control signal. The control signal is derived from a comparison of the oscillator output with the received signal. FIG. 4 shows a typical prior art receiver which makes use of a phase-locked loop. The received signal is supplied to terminal 20 and applied to a squaring circuit 22 to remove the binary phase modulation on the signal. The squared signal is at twice the original frequency and is filtered by band pass filter 26 and applied to a phase comparator 28 which determines the phase difference between the squared signal and the signals generated by local oscillator 30 which operates at twice the frequency of the received signal. A phase-difference error signal is provided at the output of phase comparator 29, filtered by loop filter 32, and used to control the oscillations of voltage controlled local oscillator 30. The output of voltage controlled oscillator 30 is halved in divider 31 and used as a local oscillator signal to demodulate the received phase modulated signal in phase detector 24. Variations on the phase locked loop of FIG. 4 are known in the art. For example, the Costas loop makes use of a quadrature output signal from voltage controlled oscillator 30, which is mixed the the received modulated signal to generate an error signal. The correct phase for the error signal is determined by reference to the received data.
From the foregoing discussion, it should be recognized that phase shift modulated signals may be easily generated by use of a simple modulator circuit, such as the FIG. 1 circuit, but, according to the prior art, complex and expensive circuits, are required to achieve demodulation of phase-shift modulated signals.
It is therefore an object of the present invention to provide a new and improved demodulator for use with phase-shift modulated RF signals.
It is a further object of the present invention to provide such a demodulator which eliminates the need for a phase-locked local oscillator for the regeneration of the unmodulated carrier.